1. Field of the Invention
The invention relates to radio communications and, more particularly, to a radio communication system, a coordinator unit and communications terminal.
2. Detailed Description of the Related Art
A large number of methods relating to when and how communications terminals obtain access to a data transmission medium are known to the person skilled in the art from communications technology, network technology and automation technology. In many cases, access to communication terminals, i.e., ‘slaves’, is allocated by a coordinator unit, i.e., a ‘master’. As the sole network user, the coordinator unit has the right to access the data transmission medium unasked. However, the communications terminals must wait for the allocation from the coordinator unit before they are allowed to access the data transmission medium. The solution known as the ‘master/slave method’ is also used in many bus systems in automation technology.
Wireless networks, i.e., Wireless Sensor Actor Networks, have recently been established in addition to “wired” bus systems for data communication between the coordinator unit and the communications terminals in automation technology. Access by the individual communication terminals to the radio resource, is again allocated by the coordinator unit. An important method with respect to access to the radio resource is time-division multiplexing in which the time is divided into individual time frames of uniform length, which are in turn divided into determinate time slots of equal length. Access to the radio resource is possible for a communications terminal only within the allocated time slot or slots within a time frame. The temporal position of the individual time slots in successive time frames is determined in relation to the start of the respective time frame.
The allocation of a time slot, which recurs in successive time frames, by a coordinator unit to a communications terminal allows deterministic transmission of data. Deterministic transmission should be taken to mean that the data are transported within a time known and determined in advance from a data source (the sender) to the destination, i.e. the data sink (the receiver).
Furthermore, the data should be transmitted within a low latency=delay time. This should, moreover, be the time within which, following the occurrence of an event which is registered by a communications terminal, this event is communicated to the coordinator unit. In the described time-division multiplexing with time frames and a time slot that is fixed for a communications terminal, the latency is substantially determined by the length of the fixed time slot and the length of the time frame. Conventional technical requirements demand latencies of a few milliseconds.